Digital Signal Processing Reference
In-Depth Information
Figure 8.1. A generic sensor pro-
cessing pipeline for CMOS sensor
processing.
8.1 CMOS Sensor Basics
A CMOS sensor will generally consist of a grid of pixel sensors,
each containing a photo-detector and an active amplifier.
CMOS sensors record the image data in grayscale. Color
information is gathered by applying a color filter over the pixel
grid. Such a color filter array (CFA) allows only light of a given
primary color (R, G, or B) to pass through
e
everything else is
absorbed, as shown in
Figure 8.2
.
This means each pixel sensor collects information about only
one color. This is important to recognise as the data for each pixel
is not composed of three colors (RGB), but of a single color. The
other colors need to be “guessed” (interpolated) by the elec-
tronics behind the CMOS sensor.
There are many types of color filter arrays
e
the most common
was invented by Dr. Bayer of Eastman Kodak. This color filter
array
e
or the Bayer mosaic as it is more commonly referred to
e
utilizes a filter pattern that is half green, a quarter red and
a quarter blue. This is based on the knowledge that the human
eye is more sensitive to green.
The resultant image data does not have all three color
components for each pixel. For some pixels we have only red
data, for some only green and for others we have only blue. To do
any meaningful video processing each pixel must have all the
color plane data. So we must process this Bayer image to get three
color planes for each pixel.
Figure 8.2. The Bayer color
lter array that
gives more green than red and blue data.
